In Situ Observation of Cirrus Scattering Phase Functions with 22° and 46° Halos: Cloud Field Study on 19 February 1998
Résumé
Observations of halos and related phenomena due to ice crystals are commonly reported from ground observations and presented in the literature. Nevertheless, ice crystal characteristics have only been poorly documented from in situ measurements performed in halo-producing cirrus with simultaneous observations of optical phenomena. Using the Polar Nephelometer, a new instrument for in situ measuring of the scattering phase function of cloud droplets and ice particles, 22 and 46 halo features have been evidenced during a cirrus uncinus cloud case study between 30C and 38C. Simultaneous microphysical measurements were made with a 2D-C probe manufactured by Particle Measuring Systems Inc. (PMS). The results show that ice crystal properties derived from 2D-C measurements do not present substantial differences when comparing cirrus cloud samples with and without halos. Consequently, the cloud scattering properties appear to be dominated by small ice particles (smaller than about 100 m), which are poorly documented with conventional PMS probes. The halo occurrences are observed in only a few cloud portions (2%), which are characterized by small horizontal scales (100-400 m). Furthermore, the observed 22 and 46 peak features are smoothed out with regard to modeling results relative to geometric pristine-crystal shape. These differences are discussed by using the new Inhomogeneous Hexagonal Monocrystal theoretical model of light scattering.
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